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20.2. Writing Editing Scripts

When you write a script that contains a series of editing actions and then run the script on an input file, you take what would be a hands-on procedure in an editor such as vi and transform it into a look-no-hands procedure.

When performing edits manually, you get to trust the cause-and-effect relationship of entering an editing command and seeing the immediate result. There is usually an "undo" command that allows you to reverse the effect of a command and return the text file to its previous state. Once you learn an interactive text editor, you have the feeling of making changes in a safe and controlled manner, one step at a time.

Most people new to "power editing" will feel there is greater risk in writing a script to perform a series of edits than in making those changes manually. The fear is that by automating the task, something will happen that cannot be reversed. The object of learning scripting with ex or sed is to understand the commands well enough to see that your results are predictable. In other words, you come to understand the cause-and-effect relationship between your editing script and the output you get.

This requires using the editor in a controlled, methodical way. Gradually, you will develop methods for creating and testing editing scripts. You will come to rely upon these methods and gain confidence that you know what your script is doing and why.

Here are a few tips:

  1. Carefully examine your input file, using grep, before designing your script.

  2. Start with a small sample of occurrences in a test file. Run your script on the sample and make sure the script is working. Remember, it's just as important to make sure the script doesn't work where you don't want it to. Then increase the size of the sample. Try to increase the complexity of the input.

  3. Work carefully, testing each command that you add to a script. Compare the output against the input file to see what has changed. Prove to yourself that your script is complete. Your script may work perfectly based on your assumptions of what is in the input file, but your assumptions may be wrong.

  4. Be pragmatic! Try to accomplish what you can with your script, but understand that it doesn't have to do 100 percent of the job. If you encounter difficult situations, check to see how frequently they occur. Sometimes it's better to do a few remaining edits manually.

If you can add to these tips with your experience, tack them on.

One additional suggestion is to use a revision control system (Section 39.4) to preserve previous versions. That makes it easy to undo your edits.

-- DD

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